1. Field of the Invention
This invention relates generally to deaerators, and more particularly to spray tray-type deaerators that are used in combined cycle power plants.
2. Statement of the Prior Art
Combined cycle power plants are well known designs for the efficient generation of electricity by utilizing both gas and steam turbines Described most simply, the process employed within combined cycle power plants uses one or more gas turbines (which are also referred to as "combustion turbines") not only to generate electricity directly through their associated electric generators, but also to assist in the generation of additional electricity through a steam turbine. That is, the hot exhaust gases issuing from the gas turbine provide waste heat which is used to generate steam to drive the steam turbine.
In this manner, the otherwise waste heat that is contained in the gas turbine exhaust gases is effectively utilized. Such combined cycle power plants also generally comprise one or more heat recovery steam generators (which are also referred to as HRSG's, or simply "steam generators") and a condenser that is associated with the steam turbine for receiving the spent steam to convert it into a condensate for supply back to the steam generator.
A typical steam generator includes a deaerator to provide the following functions The primary function of a deaerator is to "degasify" the feedwater that is used to generate steam in the steam generator. Gases, typically oxygen, that become entrained in the feedwater are removed by well known means within such deaerators. However, deaerators also perform the following secondary functions: (1) they heat the feedwater to a temperature that is sufficient to avoid corrosion of low-temperature sections of the steam generator; (2) they provide a source of water that is of a sufficient capacity and head pressure in order to satisfy the requirements of the boiler feed pumps during steady state and transient operations of the combined cycle plant; and (3) they provide a location where low level thermal energy, in the form of hot water or low pressure steam, can be effectively utilized to enhance overall efficiency of the combined cycle plant.
A spray-tray type deaerator is most frequently used in combined cycle plants With a spray-tray deaerator, deaeration is accomplished in two stages. Condensate from the condenser is first atomized by spray nozzles and heated to a point near the saturation temperature. This first stage, thus, produces the conditions and surface area necessary to allow the dissolved air to readily escape. Thereafter, additional deaeration is accomplished in the tray stage, in which the condensate flows downwardly across a series of trays while water vapor passes counter-currently to sweep the gases from the liquid. As is conventional, gases and water vapor leave the uppermost tray and flow to the spray section. Then, the water vapor is condensed while heating the incoming condensate, and the free gases are vented from the top of the deaerator.
A deaerating subsystem in a combined cycle plant is typically constructed as two separate vessel. One such vessel is the deaerator itself, while the other vessel is a storage tank. Deaerated water from the lowest tray of the deaerator flows directly into the storage tank through a downcomer to maintain a liquid level therein. The water vapor that is required for the deaerator may be obtained by extracting vapor from the space within the storage tank above this liquid level through risers, although low pressure steam from another, external source is often admitted into the deaerator for this purpose.
Spray-tray deaerators are subject to failures of their internal parts (e.g., the spray nozzles and trays) which can significantly reduce their ability to perform their primary function of degasification When failures of a combined cycle plant's deaerator occur, feedwater is then degasified by known alternate methods such as degasification in the condenser, degasification of the make-up water, or chemical scavenging by injection of hydrazine or other known chemicals that are used to remove oxygen.
However, these alternate degasification methods are not as effective or economical over the entire operating range of a combined cycle plant as traditional spraytray deaeration. They may be used for periods of time that are sufficiently short that the impact of boiler tube corrosion and increased chemical costs are insignificant, but for extended periods of time, the increased costs and potential risks that are associated with operation of a combined cycle plant without a deaerator dictate that a failed deaerator must be removed from service for repairs.
Such deaerator repairs, however, normally require shutdown of the entire combined cycle plant. This is because feedwater cannot be provided at a suitable temperature, capacity, and pressure with the deaerator out of service, even though sufficient degasification can be temporarily provided by the alternate methods discussed above.